To produce ply adhesion between layers composed of tissue paper to manufacture e.g. a hygienic tissue product, such as kitchen paper, toilet paper, napkins, handkerchiefs, face wipes and the like, use has so far been made of either glue or mechanical tools so as to produce edging or marginal embossing, whereby the layers were considerably compressed in the region of the edging seam or marginal embossing and joined together in this way. These techniques are still in common use today. The tools used for this purpose nevertheless leave clearly visible marks on the tissue surfaces which are frequently undesirable, e.g. in the case of edging, unless the tissue paper is provided with visible surface embossing in which adhesion between the layers is simultaneously produced by means of embossed printing, as is usually the case for marginal embossing of e.g. handkerchiefs or napkins.
The invention""s object (problem) is to design a method and an apparatus which make it easier to produce good ply adhesion between the layers of the tissue product manufactured therewith, while largely maintaining the tissue paper""s softness, strength and absorbency even if there is a fairly large number of layers, though without leaving any clearly visible marksxe2x80x94which users perceive to be negativexe2x80x94on the outwardly showing tissue surfaces of the tissue product""s outer layers.
In accordance with the invention""s method, this object is solved by the following procedural steps:
bringing together at least two layers of tissue paper in a roll gap; and
imprinting an irregularly rough surface structure, located in the roll gap and as exhibited e.g. by abrasive paper (sand paper), onto one or both sides of the tissue paper;
and in accordance with the invention""s apparatus, this object is solved by:
a pair of rolls, between the rolls of which a through-flow pressing gap exists; and
an irregularly rough surface structure following the periphery of the rolls; this surface structure is present on one or both sides of the pressing roll gap and is similar to that of sand paper.
The method and apparatus enjoy the advantage that glue can be dispensed with in order to produce ply adhesion and that the roll surface""s mechanical influence on the tissue surface is hardly visible or is invisible. On the other hand, a visible desirable irregular pattern which can be utilized to design new products can be produced by selecting certain marginal conditions. After ply adhesion has been produced in this way, the paper acquires a surface similar to the skin of a peach, making the paper very soft to the touch.
Imprinting occurs at a pressure sufficient to produce inter-fiber bonds in the region of most of the roughness peaks acting upon the fiber web of the individual layers of a multi-ply tissue product; such fiber bonds are produced between contiguous surfaces of fibers close to the surface of adjacent outsides of the individual layers of the tissue product.
Ply adhesion can also be produced in more than one roll gap and roll gaps can be provided in succession, whereby the pairs of rolls can operate at varying pressure, e.g. with increasing pressure. The pairs of rolls can also be provided with a different pattern. If the product has more than two layers, they can be guided through different pairs of rolls/roll gaps.
In addition to the main process claim and main device claim, other advantageous embodiments are obtained from their respective dependent claims.
The invention also relates to a use of sand paper for the production of ply adhesion between layers of tissue paper by imprinting the sand paper""s surface structure into a tissue paper composed of at least two layers when manufacturing a multi-ply tissue product intended for final consumption.
To produce ply adhesion, the aim is to make as many xe2x80x9croughness peaksxe2x80x9d available as possible per unit of area of the tissue papers to be adhered together, whereby each such xe2x80x9croughness peakxe2x80x9d must have sufficient height, inherent strength and anchoring strength to be able to penetrate deeply enough into the paper. The shape, hardness, inherent and anchoring strength of each xe2x80x9croughness peakxe2x80x9d have to be such that as high a specific surface pressure as possible arises in the region of the xe2x80x9croughness peakxe2x80x9d end faces acting on the fiber structure of the tissue paper in a pressing gap under load; this surface pressure is able to deform the fibers of a tissue paper which is exposed to this surface pressure in at least two superimposed layers such that the surfaces of adjacent fibers come into such close contact with one another that inter-fiber bonds which in their totality result in the finished tissue product""s ply adhesion arise locally.
A simple, hitherto unadopted approach to this end is for example the use of abrasive grain materials e.g. made of electrocorundum, silicon carbide or industrial diamond in a macro granulation range of P 40 to P 240 (German DIN standard), anchored e.g. on suitable flexible abrasive supports or directly on the surface of a solid roll made of metal or a suitable metal shell that can be mounted onto a roll core. Other feasible embodiments occur as a surface structure similar to sand paper on, or in the form of, a ceramic shell which can also be mounted onto a roll core and endures corresponding loads. Suitable solid rolls, e.g. solid ceramic rolls with a surface structure that resembles sand paper, can also be used.